#include <stdio.h>
#include <math.h>


typedef unsigned long int moj;

int loop( moj ,moj,moj, int);

int main(void) {
    moj a,b,c,k;
    do {
    scanf("%ld%ld%ld%ld",&a,&b,&c,&k);
    if (!((a==0) && (b==0) && (c==0) && (k==0))) loop(a,b,c,k);
    }while (!((a==0) && (b==0) && (c==0) && (k==0)));
    return 0;
}

int loop(moj a,moj b,moj c, int k) {
moj zvysok = 0;
int forever = 0;
unsigned long int hlp,hlp1;
moj vysl = 0;
moj dvanak = 0;
dvanak = ceil(exp(k*log(2)));
if (a>b) {
    hlp = dvanak - a;
    hlp1 = c;
    zvysok = hlp % hlp1; 
    vysl = (dvanak - a) / c;
    hlp = b-zvysok;
    if ((hlp % hlp1)==0) vysl = 1 +vysl + ((b-zvysok) / c);
    else forever = 1;
    }
    else {
    hlp = b-a;
    if ((hlp % 2) == 0) vysl = (b-a)/2;
    else {
    hlp = dvanak - a;
    hlp1 = c;
    zvysok = hlp % hlp1; 
    vysl = (dvanak - a) / c;
    hlp = b-zvysok;
    if ((hlp % hlp1)==0) vysl = 1+vysl + ((b-zvysok) / c) ;
    else forever = 1;
    }
    } 
if (forever) printf("FOREVER\n");
else printf("%ld\n",vysl);
return 0;
}